As a type of wheels for supporting tires required for automobiles to travel on, there have widely been used two-piece wheels comprising a vehicular wheel rim (hereinafter also referred to simply as “rim”) in the form of a hollow cylindrical body and a disk inserted in the wheel rim, the wheel rim and the disk being joined to each other by MIG welding or the like.
The rim is manufactured as follows: First, the end faces of an elongate rectangular plate are brought into abutment against each other, and thereafter the abutting end faces are joined to each other by resistance welding or the like, thereby forming a hollow cylindrical body. Then, the hollow cylindrical body is rolled by a multi-step rolling process, forming a recess called a drop portion in a substantially central region of an outer circumferential wall of the hollow cylindrical body. Thereafter, the ends of the hollow cylindrical body are bent into curled portions.
In order to prevent air from leaking out of the tire mounted on the wheel and also to prevent the beads of the type from falling into the drop portion at the center of the rim, hump portions are formed on the hollow cylindrical body, thereby completing the rim.
To form the hump portion, it has been proposed in Patent Document 1 to roughly form a hollow cylindrical workpiece to extend an outer circumferential wall surface thereof, and press a die having a recess against the outer circumferential wall surface to finish the outer circumferential wall surface for thereby further extending the outer circumferential wall surface and causing the material of the outer circumferential wall surface to flow into the recess to raise the outer circumferential wall surface into the hump portion.
Patent Document 2 discloses a process of manufacturing a rim by inserting a first roller having a ridge on a side circumferential wall thereof into a hollow cylindrical body, placing a second roller having a recess outside of the hollow cylindrical body, displacing the first roller and the second roller toward each other, and finally pressing the ridge of the first roller against an inner circumferential wall surface of the hollow cylindrical body to raise an outer circumferential wall surface thereof into a hump portion. The raised outer circumferential wall surface of the hollow cylindrical body enters the recess of the second roller.
A disk is then inserted into the rim thus manufactured, and the disk and the rim are joined to each other by MIG welding or the like, thereby forming a wheel.
Patent Document 1: Japanese Laid-Open Patent Publication No. 10-71443
Patent Document 2: Japanese Laid-Open Patent Publication No. 2-299733